1. Field of the Invention
The present invention relates to an inkjet printing apparatus and a method for correcting a printing position shift. More particular, the present invention relates to a method for correcting a printing position shift in an inkjet printing apparatus including a plurality of nozzle arrays that are arranged side by side and each extend in a width direction of continuous paper such as a roll of paper or a web in order to print images on the continuous paper.
2. Description of the Related Art
In the full-line type color inkjet printing apparatus, a plurality of nozzle arrays ejecting different inks are arranged at predetermined intervals in the conveying direction of a print medium. In the type of the printing apparatus, to print dots on the same position of the print medium, timing to eject ink is shifted with respect to each nozzle array. There is known a method of, in order to adjust the ink ejecting timing, adding null data which is data on no ink ejection to print data to be printed by each nozzle array, and varying the amount of addition with respect to each nozzle array.
In this case, typically, the null data is set in predetermined bit units for CPU to facilitate processing. Therefore, the interval in each nozzle array is set to allow the ink ejection timing to be adjusted, which makes it difficult to determine an interval in each nozzle array in an arbitrary manner.
Japanese Patent Laid-Open No. 2004-330771 discloses a method of changing addresses for starting reading null data in accordance with positions of the nozzle arrays while different amounts of null data are added on a nozzle array basis because the interval of each nozzle array can be set in an arbitrary manner.
Even if ink ejection timing is adjusted with the method according to Japanese Patent Laid-Open No. 2004-330771, when the speed of ink ejection is varied from nozzle array to nozzle array, the print positions of dots are shifted. To prevent this, Japanese Patent Laid-Open No. 2007-152853 discloses a printing apparatus which measures the ejection speed of ink droplets on the basis of a cumulative number of ink droplets and performs registration adjustment when a speed change is detected.
The printing apparatus uses conveying unit to convey a print medium, in which the coefficient of friction between a conveying roller which is the conveying unit and the print medium may possibly vary because a change in surface state caused by adhesion of paper powder to the surface of the conveying roller, the moisture contents of the print medium, environmental conditions in the printing apparatus and the like. This may possibly change the amount of conveying the print medium. A change in the amount of conveyance when an image is printed on the continuous paper may occur, for example, when the print medium cut after a continuous image has been printed on it is given as one set, at the start of printing on each of a plurality of sets to be output or during the process of printing the continuous image. In this manner, if the amount of conveyance of the print medium per unit time is changed, print positions shift between print heads.
In the construction in Japanese Patent Laid-Open No. 2004-330771, an address to start reading print data is fixedly determined on a nozzle array basis with reference to the position of the nozzle array. Because of this, when the amount of conveyance of the print medium per unit time is varied by conditions of the conveying unit and the print medium, appropriate adjustment for ink ejection timing is difficult to be performed.
Further in the construction of Japanese Patent Laid-Open No. 2007-152853, since registration adjustment is performed at the timing when a change in ink ejection speed is detected, when the amount of conveyance of the print medium per unit time is varied by conditions of the conveying unit and the print medium, the printing position shift is difficult to be addressed.